More random values for oAuth1 verifier

The oAuth1 verifier was generated as a random number ranging from
1000 to 9999. This small range of numbers is vulnerable to
brute-force attacks as described in CWE-330. The verifier is now
a 8-character long alphanumerical string, a good compromise between
security against guessing and ease of use.

SecurityImpact
Change-Id: Ibe4a2e57a02c261d85ba6c0d61696f134c54443e
Closes-Bug: #1236675
Merge "More random values for oAuth1 verifier"