camera2 vndk: Add ACameraMananger_getTagFromName.

In some situations, clients of the vndk might need to be able to query
tags at runtime, based on tag names. For example, when client hals are
de-coupled from the camera HAL.

Bug: 131093919

Test: AImageReaderVendorTest
Test: Modify AImageReaderVendorTest to retrieve vendor tags given their
      names, using ACameraMetadata_getTagFromName

Change-Id: I1cdec5b154037185e99d29be2c6890e4fdc4a32a
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>