mediaserver: add more hwbinder threads

Since we're now using HIDL IGraphicBufferSource in CCodec, the
existence of a callstack with mixed binder and hwbinder calls can cause
a stall when all threads are in use. (b/35283480) The recommended
workaround was to increase the number of threads. This CL does that.

Test: atest CtsMediaTestCases
Test: atest CtsCameraTestCases

Bug: 153828976
Change-Id: I37f02b6237ad032e311a8219b209713d2e4463a6