CameraExtensionProxy: Cache vendor id map

Extensive camera characteristics queries can
be expensive on some devices. The vendor ids
are not likely to change once set by camera
service so their values can be cached to
speed up the extension proxy service
initialization.

FLAG: EXEMPT trivial optimization
Bug: 356964467
Test: Manual using camera extension sample application,
Camera CTS

Change-Id: Ic450cc593d1afee7351f4ab00d059b58c3d30855