Fix Context leak in XrInstanceManager and scope application context to OpenXR

Fixed the following leaks:
 - XrInstanceManager was keeping a reference to initContext, removed the
   cache
 - OpenXrDeviceCapabilityProvider was holding a reference to context as
   part of the interface, passed the ApplicationContext instead
 - OpenXrInstanceManager was passing activity context to natives which
   was holding it as a global JNI reference, changed to
   ApplicationContext

Used double-checked locking in XrInstanceManager to ensure thread-safe,
single initialization of provider.

Test: test app and activity leak test
Relnote: Removed Activity context leak in `XrDevice`.
Bug: 553546744
Change-Id: I71996450b13c5af7e9b1faf92003547d144147b2