Declare Apple platform requirements and align swift-android-native source

[colemancda]

Two manifest fixes found while building a downstream app (BluetoothExplorer) that depends on this
package transitively.

Declare Apple platform requirements

The package declared only .macOS(.v15). SwiftPM defaults unspecified platforms to iOS 12, while
Socket/Bluetooth and friends require iOS 13 — so any Apple-platform resolve of a graph
containing this package failed:

the library 'AndroidLooper' requires ios 12.0, but depends on the product 'Socket'
which requires ios 13.0; consider changing the library 'AndroidLooper' to require
ios 13.0 or later, or the product 'Socket' to require ios 12.0 or earlier

That was 4 of 15 such errors in the downstream app; the rest came from AndroidBluetooth
(PureSwift/AndroidBluetooth#…) and swift-java.

Now declares .iOS(.v13), .watchOS(.v6), .tvOS(.v13) to match the dependencies. This package only
ever builds for Android, but SwiftPM validates platform requirements across the whole graph
regardless of .when(platforms:) conditions, so the declaration has to be there.

Align the swift-android-native source

This package pointed at swift-android-sdk/swift-android-native @ main, while
MillerTechnologyPeru/skip-android-bridge points at
MillerTechnologyPeru/swift-android-native @ feature/pureswift. SwiftPM resolves by package
identity, so a graph containing both is unresolvable:

error: swift-android-native is required using two different revision-based requirements
(feature/pureswift and main), which is not supported

Now points at the same fork/branch the bridge uses.

Note: this depends on MillerTechnologyPeru/swift-android-native#… (merging current upstream
main into feature/pureswift), because the fork was 87 commits behind and did not yet export
AndroidManifest, AndroidFileManager, AndroidInput or AndroidHardware, which this package
requires. Merge that first.